1. Field of the Invention
The invention refers to a microsensor with a micro-electromechanical sensor element and an integrated circuit for measuring, calibration and compensation electronics, whereby the sensor element has an electric connection to the integrated circuit (IC).
2. Prior Art
Microsensors of the type mentioned above are used for a wide range of applications. In addition to proximity switches and acceleration sensors, for example semiconductor pressure sensors have already become known, in which a sensor element and a carrier of different materials are joined by soldering. As a sensor element for such pressure sensors, silicon membranes have already been suggested, whereby various degrees of compression can result due to the different e-modules of the material used. Pressure leads to deformation, and use of the piezoresistant effect of resistors diffused into silicon membranes for measuring pressures up to several 100 bar is already known. In the known microsensors, the electrical sensing is achieved using bonded pads and bonded lines or bonded wires, whereupon the evaluation is performed using integrated circuits via bonded wires. Such circuits can include conventional measuring, calibration and compensation electronics. Due to the relatively long lines between the components to be joined, there are relatively high parasitic capacities of bonded pads and wires, particularly in capacitive sensors, which cover most of the sensor""s wanted signal, thus reducing the sensitivity.
Soldered joints are common in microelectronics, and in particular the electric and mechanical joining of ICs using soldering islands is known. Furthermore, flat soldered joints are known with which passive silicon components are soldered onto a carrier to produce the piezoresistant sensors mentioned above. Subsequently, the surface of the membrane must be contacted with conduction paths, whereby these contacts face away from the soldered joint side, which in turn leads to relatively long line lengths.
The aim of this invention is to create a microsensor of the type mentioned above, which is suitable for a wide range of different applications and which in particular reduces the influence of parasitic capacities to a great extent, so that in the case of capacitive sensors resolutions in the atto-Farad range are quite easily possible. To solve this problem, the microsensor according to the invention consists mainly in the fact that the micro-electromechanical sensor element is arranged directly on the integrated circuit, and is joined with accurate positioning and electric conductivity by a circulating soldered joint. Because the micro-electromechanical sensor element is arranged directly on an integrated circuit (IC) with accurate positioning and electric conductivity is provided by a circulating soldered joint, the mechanical connection provided by the soldered joint at the same time provides the electric connection with considerable shortening of the conduction path lengths, whereby the switching components required for measurement and evaluation can be arranged in the integrated circuit, i.e. directly beside the micro-electromechanical sensor element. Such a design permits the creation of highly sensitive systems that are at the same time protected from corrosion, whereby the costs of manufacture can be reduced significantly in comparison with the existing multi-component sensor systems. In such a design, the advantage of a modular structure is achieved initially by simple exchange of the micro-electromechanical component, whereby a uniform manufacturing process for various sensors using tried and tested, reliable ICs can be used. The mechanical part can be manufactured easily and reliably in terms of technology, and the connecting technology can be kept the same for various types of sensors.
The design according to the invention thereby offers the advantage that the micro-electromechanical sensor element is designed and circuited as a capacitor plate. In such a design, it is sufficient for the micro-electromechanical sensor element to be made of conductive silicon, for which conventional etching techniques are available. Subsequently, such a sensor element can be used for various parameters, whereby the preferred design is such that the micro-electromechanical sensor element is designed as an electrically conductive flexural bar for measuring distances, slopes, acceleration and/or rotating speeds. In particular the measurement of rotating speeds can be made much more sensitive with the design according to the invention, since the design can be made in such a way that radial acceleration from the measurement can be eliminated to a large extent. During rapid torsion, the Koriolis force leads to bending of such a rotating sensor element, and thus to a reduction of the dielectric gap between opposite capacitor components or capacitor plates, so that for example only the Koriolis force is measured as change in capacity, thus permitting high sensitivity and accurate measurement of the turning rates. The soldered joint according to the invention using a circulating soldered joint allows the soldered joint at the same time to be gastight, thus reducing the risk of corrosion significantly and creating the possibility to at least partly evacuate the cavity and thus the gap between the opposite capacitor plates, which acts as dielectric space, so that the extent of gas attenuation of the sensor can be influenced in a simple manner. The high sensitivity can be achieved quite simply by designing the integrated circuit in such a was that there is a capacitor surface for a reference capacity beneath the micro-electromechanical flexural bar in the area of the projection of the link for the unilaterally mounted flexural bar. With such a design, a reference capacity for electronic evaluation and compensation is at the same time provided and thus the compensation and evaluation of signals is facilitated considerably.
In gas-filled micro-electromechanical components, the electrically conductive flexural bar is generally gas-attenuated to an over-critical degree. In such cases, the sensitivity can be increased considerably by arranging an electrode for applying an electrostatic force to the flexural bar on the IC within the projection of the flexural bar. An electrostatic force that is not directly correlated with the applied voltage makes it possible to stabilise the flexural bar and at the same time to increase the useful frequency range of the oscillation, whereby for example pulse-width modulated voltage surges can be applied with unchanged voltage to the electrode in order to provide the electrostatic force.
A particularly cost-efficient production of the microsensor is possible due to the fact that the micro-electromechanical component is designed as a back-etched cuboid of electrically conductive Si, and is connected to the IC over a spacer by a gastight, circulating soldered joint. The preferred design of the circulating soldered joint as a gastight joint thereby provides the possibility to at least partly evacuate the corresponding cavity, or to fill it with other gases, thus improving the sensitivity or the measurable frequency range. In the case of pressure sensors, an according pressurization of the flexural bar through appropriate drill holes or break-throughs in the mechanical component is naturally possible, whereby the electrically conductive membrane can be designed as a multi-laterally mounted flexural bar and in particular as a gastight membrane, rather than a unilaterally mounted flexural bar. In such a case, the circulating soldered joint can again be gastight, since it is sufficient to provide the relevant gas entries on the side of the membrane facing away from the soldered joint.
To reduce the risk of corrosion, the design is advantageously so that the cavity formed between the IC and the micro-electromechanical component is evacuated.